1. Field of Invention
The present invention relates generally to microstructures and nanostructures, and, more particularly, to corrugated and nanoporous microspheres/nanospheres, and methods of synthesizing the same.
2. Description of Prior Art
The synthesis of inorganic nanomaterials with controllable sizes, shapes, and structures has become increasingly important in modern inorganic materials chemistry today. These materials exhibit a wide range of unique physical, chemical, surface, electronic and optical properties associated with their sizes and shapes. They thus have potential for catalysis, separation, chromatography, surface enhanced Raman spectroscopy and biological diagnostic applications; as well as for fabrication of various electrical, photovoltaic, photonics, magnetic, microelectronics, chemical sensor and optical devices. Thus far, most synthesis methods have concentrated on smooth, spherical and symmetrical nanomaterials, mainly because their synthesis is simpler and their size is easier to control. Nanoparticles with non-spherical or non-symmetrical shapes are known to possess several properties that are unique compared to their spherical counterparts, while their controlled synthesis is often met with considerable challenges. For instance, the catalytic activity of noble metal nanocrystals depends on their shapes in addition to their sizes. Unique physical properties, such as optical and electronic as well as magnetic flux trapping and photoluminescence, can also be obtained from shaped and anisotropic nanomaterials. Furthermore, it was proven that non-symmetrical and non-spherical inorganic, organic, and biological nanostructures can self-aggregate into rather unique structures that their corresponding spherical counterparts are not capable of forming Consequently, the self-aggregates from non-spherical nanomaterials can produce unusual properties as well as unique “hard templates” that can be useful for generating other asymmetric nanostructures and photonics band gap materials for microphotonics and microelectronics applications.
The Stöber synthesis, which was first reported in 1963, has long been the method of choice for making silica microspheres. The silica microspheres that result from the Stöber method have rather symmetrical or spherical shape and a smooth surface. Many researchers have demonstrated that these silica microspheres have potential applications in areas ranging from chromatography to catalysis. For instance, by using the silica microspheres, various metal supported catalysts, metal nanoshells for biological applications, and hollow and core-shell nanomaterials have been successfully synthesized. The recent advances in the field of photonics have also resulted in renewed interest in the development of synthetic methods to monodisperse silica microspheres and their self-assembly into opal and inverse opal structures. Silica microspheres, particularly those with monodisperse size, can pack into perfect colloidal crystals, which can then be infiltrated with various precursors to produce so-called photonics band-gap materials. The resulting photonics band gap materials have interesting optical light trapping properties that are useful for photonics applications. However, since the silica microspheres synthesized by the Stöber method or some variations of the Stöber method often have smooth surfaces, the complete infiltration of their perfectly packed colloidal crystal structures with monomers, chromophores, polymers and other molecules remains to be problematic. Consequently, the formation of defect sites and void spaces in the resulting opal and inverse opal structures as well as in photonics band gap materials is often too common.
Recently, upon etching gold nanoparticles (AuNP) sandwiched between a silica microsphere and a silica shell with aqueous KCN solution, it has been observed that a higher concentration of KCN solution etches the silica shell and produces some silica/AuNP/silica core-shell-shell nanospheres containing a corrugated surface. Furthermore, while the etching of solid glass substrates and metal oxide microspheres, including silica microspheres, by various strong bases and HF solutions is already known, it has been widely used for nanopatterning solid state substrates or for complete dissolution of silica to create hollow nanostructures. For instance, by utilizing the commonly used etchants such as HF solutions or strongly basic KOH and NaOH solutions, complete dissolution of silica nanostructures was achieved. Silica in strongly basic solutions undergoes quick dissolution via the hydrolysis of its siloxane bonds while silica in HF solutions form soluble tetrafluosilicate species. In addition to these wet-etching processes, other physical methods involving etching with plasma, molecular beam epitaxy (MBE), and laser ablation can be used to etch silica or other metal oxides. By using the latter methods, many nanoelectronics and optical devices have also been fabricated.